The increasing demand for improved fiber optic components in telecommunications systems and in medical devices has led to the need for novel glasses. The telecommunications industry utilizes waveguide amplifiers to intensify optical signals that have been attenuated along the length of a fiber optic communication path. Optical communication systems usually operate in two separate bands, namely at about 1300 nm and at about 1550 nm. Typically, these fiber optic components utilize glasses which have been doped with a rare earth element. Doping with rare earth elements generally enables the production of glass materials capable of efficient, low-loss optical transmission and amplification at desired fluorescence bands. For example, erbium has been used as a dopant for amplifiers operating in the 1550 nm band, whereas neodymium, dysprosium, or praseodymium are used as dopants in amplifiers operating in the 1300 nm band. U.S. Pat. No. 3,729,690 to Snitzer describes a glass suitable for use as a laser comprising a host material that contains a fluorescent trivalent neodymium ingredient. U.S. Pat. No. 5,027,079 to Desurvire et al. describes an optical amplifier comprising a single mode fiber that has an erbium-doped core. Also, U.S. Pat. No. 5,239,607 to da Silva et al. describes an apparatus and method for flattening the gain of an optical amplifier that utilizes an erbium-doped silica fiber having a germanosilicate core. U.S. Pat. No. 5,563,979 to Bruce et al. describes an erbium-doped planar optical device whose active core includes a mixture of oxides such as lanthanum and aluminum oxides.
Suitable glasses which may be used in optical components such as those described above must be stable (i.e., resist devitrification). Preferably, the glasses are formed using conventional glass-forming techniques which do not require additional production costs and are compatible with currently available cladding materials. Finally, the glass must possess certain characteristics. One characteristic, as it pertains to use as an optical amplifier, concerns the gain measured against the width of the amplification band (i.e., gain curve). It is preferable for optical amplifiers to have a broader, flatter gain curve. However, many oxide glasses do not display a gain curve which is sufficiently flat (i.e., less than ten percent gain deviation) over a broad amplification band (i.e., greater than 32 nm).
Transparent glass ceramics which exhibit ferro-electric properties are desirable for their use in electro-optical devices of the type disclosed in U.S. Pat. No. 3,069,973 to Ames and U.S. Pat. No. 3,467,463 to Borrelli et al., and acousto-optical devices such as, for example, modulators, laser Q-switches, and/or deflectors. Glass ceramics with sufficiently high dielectric properties at room temperature are also useful in electrical devices such as capacitors, electro-luminescent cells, etc.
Generally, glass ceramics are transparent when their constituent crystalline particles are so small in size that they produce no effective light scattering even at the short wavelengths of the visible spectrum, or when the refractive index difference between the glass phase and crystalline phase is sufficiently small. Because glass ceramics containing ferro-electric crystals generally have crystalline phases with a much higher refractive index than the glass phase thereof, the crystal size becomes the determining factor for transparency of the resulting glass ceramic.
U.S. Pat. No. 3,114,066 to Allen et al. discloses a transparent, high dielectric glass ceramic material comprising 5-25 wt. % SiO.sub.2, 50-80 wt. % Nb.sub.2 O.sub.5, 0-20 wt. % Na.sub.2 O, and 0-31 wt. % BaO. The crystal lattice formed by the composition of Allen et al. is described as an "oxygen octahedral" lattice. Allen et al. also discloses the substitution of Na.sub.2 O and BaO with other modifiers (e.g., oxides of mono-, di-, and tri-valent cations).
U.S. Pat. Nos. 3,785,833, 3,984,251, and 4,017,317 to Rapp disclose various glasses and glass ceramics of the Na.sub.2 O--K.sub.2 O--Nb.sub.2 O.sub.5 --SiO.sub.2, Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2, and Na.sub.2 O--Li.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 systems. In particular, the Na.sub.2 O--K.sub.2 O--Nb.sub.2 O.sub.5 --SiO.sub.2 system includes 23-38 mole % SiO.sub.2, 23-47 mole % Nb.sub.2 O.sub.5, 13-30 mole % Na.sub.2 O, and 9-22 mole % K.sub.2 O, where the Na.sub.2 O to K.sub.2 O ratio is at least 0.7 and the (Na.sub.2 O+K.sub.2 O) to Nb.sub.2 O.sub.5 ratio is from 0.8 to 1.8. The Na.sub.2 O--Ta.sub.2 O.sub.5 --SiO.sub.2 system includes 37-55 mole % SiO.sub.2, 23-35 mole % Ta.sub.2 O.sub.5, and 20-33 mole % Na.sub.2 O. The Na.sub.2 O--Li2O--Ta.sub.2 O.sub.5 --SiO.sub.2 system includes 27-45 mole % SiO2, 30-45 mole % Ta.sub.2 O.sub.5, and 20-35 mole % Li.sub.2 O+Na.sub.2 O.
U.S. Pat. No. 3,785,834 to Rapp discloses glasses and glass ceramics of the R.sub.2 O--RE.sub.2 O.sub.3 --Nb.sub.2 O.sub.5 --GF system, where R is an alkali metal oxide, RE is a rare earth metal oxide (including other trivalent cations), and GF is a glass former, such as SiO.sub.2, GeO.sub.2, or P.sub.2 O.sub.5. The composition used to form the glasses and glass ceramics includes 20-45 mole % SiO.sub.2, 34-50 mole % Nb.sub.2 O.sub.5, 7-10 mole % RE.sub.2 O.sub.3, and 14-20 mole % R.sub.2 O. The glass ceramics are preferably composed of a crystal phase having crystals with cubic perovskite or tetragonal tungsten-bronze crystal structures.
U.S. Pat. No. 3,573,939 to Beall discloses transparent glass ceramic materials containing 20-55 wt. % SiO.sub.2, 2-10 wt.% Al.sub.2 O.sub.3, 3-6 wt.% Li.sub.2 O, and 40-70 wt.% Ta.sub.2 O.sub.5 +Nb.sub.2 O.sub.5, where Nb.sub.2 O.sub.5 may be up to 10 wt. %. Beall also discloses that such transparent glass ceramics contain a perovskite structure. However, an analysis of these glass systems using conventional X-ray diffraction techniques suggests that the crystal structure is actually ilmenite, not perovskite. The transparency of such LiTaO.sub.3 --SiO.sub.2 --Al.sub.2 O.sub.3 glass ceramics has been shown to be more dependent upon the presence of Al.sub.2 O.sub.3 than on the ratio of glass forming components (e.g., SiO.sub.2) to crystal forming components (e.g., LiTaO.sub.3). Ito, S., et al., "Transparency of LiTaO.sub.3 --SiO.sub.2 --Al.sub.2 O.sub.3 Glass-Ceramics in Relation to their Microstructure," J. Mat. Sci. 13:930-38 (1978).
The present invention is directed to glasses and glass-ceramics which overcome the above-noted deficiencies in the art.